BIOMEDICAL APPLICATIONS VI: CARBON NANOTUBES AS BIOSENSING AND BIO-INTERFACIAL MATERIALS - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 187

caused an electrostatic force downward on the CNTs. This force controlled
the tension of the CNTs and resulted in guitar-string-like oscillation. When
a time-varying voltage was added, a driving frequency was produced. The
transistor properties of semiconducting and small-band-gap semiconducting
CNTs made it convenient to detect the driving force and driving frequency,
which could be monitored by measuring the conductivity. Such a device can
be used to transduce very small forces into electric signals.
7.2.2.2.2 Chemical sensors
The special atom structure and electronic structure make CNTs suitable
to develop novel devices for the detection of various types of chemicals.
Applications of CNT-based sensors on detection of both small molecules and
macromolecules have been explored.
CNTs have been shown to be extremely sensitive to chemical gas. The
electronic properties of nanotubes, including the electrical resistance R ,
thermoelectric power S (voltages between junctions caused by interjunction
temperature differences) and local density of states N ( E ), are exceedingly
sensitive to environmental conditions, such as gas exposure. Collins et al. 20
reported that the exposure to air or oxygen dramatically inluences the
nanotubes' electrical resistance and thermoelectric power, which can be
easily measured through the electric signal. In addition, these electronic
parameters can be reversibly tuned. In this study, the resistance of an SWNT
sample, with the surrounding medium cycled between vacuum and air, was
detected. A rapid and reversible change in the SWNT resistance occurred
in concomitance with the changing environment. The resistance of the CNT
was higher in vacuum and decreased by 10-15% when it was surrounded by
air. Moreover, the thermoelectric power S also showed a high sensitivity to
chemical gas exposure. When CNTs were exposed to air, their thermoelectric
power was a constant value and with a positive magnitude near 120 mV/K.
However, when oxygen was gradually removed from the chamber containing
the CNTs, the thermoelectric power changed continuously from positive to
negative, with a inal equilibrium value of ~210 mV/K. When oxygen was
reintroduced into the chamber, the thermoelectric power again reversed sign
and once again became positive. These results indicated CNTs as sensitive
sensors for oxygen.
Kong et al. 21 reported using a single SWNT-based transistor to detect
NO 2 and NH 3 gases. Gas-sensing experiments were carried out by exposing
a semiconducting SWNT sample in lowing diluted NO 2 (2-200 parts per
million) or NH 3 (0.1-1%) and monitoring the resistance of the SWNTs. Upon